title: G-actin sequestering protein thymosin-β4 regulates the activity of myocardin-related transcription factor.
abstract: Myocardin-related transcription factors (MRTFs) are robust coactivators of serum response factor (SRF). MRTFs contain three copies of the RPEL motif at their N-terminus, and they bind to monomeric globular actin (G-actin). Previous studies illustrate that G-actin binding inhibits MRTF activity by preventing the MRTFs nuclear accumulation. In the living cells, the majority of G-actin is sequestered by G-actin binding proteins that prevent spontaneous actin polymerization. Here, we demonstrate that the most abundant G-actin sequestering protein thymosin-β4 (Tβ4) was involved in the regulation of subcellular localization and activity of MRTF-A. Tβ4 competed with MRTF-A for G-actin binding; thus, interfering with G-actin-MRTF-A complex formation. Tβ4 overexpression induced the MRTF-A nuclear accumulation and activation of MRTF-SRF signaling. The activation rate of MRTF-A by the Tβ4 mutant L17A, whose affinity for G-actin is very low, was lower than that by wild-type Tβ4. In contrast, the β-actin mutant 3DA, which has a lower affinity for Tβ4, more effectively suppressed MRTF-A activity than wild-type β-actin. Furthermore, ectopic Tβ4 increased the endogenous expression of SRF-dependent actin cytoskeletal genes. Thus, Tβ4 is an important MRTF regulator that controls the G-actin-MRTFs interaction.
authors.affiliation: Department of Neuroscience, Osaka University Graduate School of Medicine, 2-2 Yamadaoka, Suita, Osaka 565-0871, Japan. tsuyo@nbiochem.med.osaka-u.ac.jp
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Morita
authors.forename: Ken'ichiro
authors.initials: K
authors.lastname: Hayashi
mesh_terms: D000199:Actins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000818:Animals; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D051379:Mice; D041681:NIH 3T3 Cells; D015398:Signal Transduction / Q000502:physiology; D013347:Subcellular Fractions / Q000378:metabolism; D013947:Thymosin / Q000378:metabolism / Q000502:physiology*; D015534:Trans-Activators / Q000037:antagonists & inhibitors / Q000378:metabolism* / Q000502:physiology; D000099201:Thymosin beta(4)
chemical_list: D000199:Actins; D013947:Thymosin; D015534:Trans-Activators; D000099201:Thymosin beta(4); C511173:Mrtfa protein, mouse
keywords: Actin cytoskeleton; F-actin; G-actin; MRTF; Myocardin; SRF; Serum response factor; Thymosin-β4; Transcription factor; Tβ4; filamentous actin; monomeric globular actin; myocardin-related transcription factor; serum response factor; thymosin-β4